During one of these rescue attempts, Spirit churned up the soil and uncovered an intriguing layer of bright, fluffy soil. Mission managers had the rover take a closer look, and they discovered that the layer is in sulfates, minerals known to form on Earth only in the presence of liquid water.

Spirit and its twin rover Opportunity have discovered sulfate-rich soil in other regions of Mars before, said Ray Arvidson, a planetary scientist at the Washington University in St. Louis and a member of the rover science team.

But because most missions are scheduled in advance, the latest discovery marks the first time one of the robots has been stationary long enough to study sulfates in detail.

Snow on Mars

Spirit's data revealed that the newfound sulfates are likely evidence of past "wet eruptions" on Mars, when lava and sulfur-rich steam spewed from volcanic vents dotting the landscape billions of years ago.

But the crater contains a clue that liquid water continues to be active on Mars, at least in the long term: The soil is full of iron sulfate covered with a thin crust of calcium sulfate.

"This is the first time we've established this kind of layering," Arvidson said.

The layers support the theory that Mars's equator experiences a long-lasting snowfall every several hundred million years or so, when Mars's axis naturally tilts and one of its poles faces the sun.

When this happens, water ice at the sunny pole sublimates—turns directly from a solid to a gas—and it snows at the equator, where the temperature has dropped.

Mars is too cold and its atmosphere too thin for liquid water to exist on its surface today. But during the periodic polar tilt, dark, heat-trapping soils at the equator allow the bottom layers of accumulated snow to melt into a liquid.

The water then mixes with the sulfate-rich soil, where it dissolves the iron sulfate and carries it deeper underground, leaving calcium sulfate on top.

Equatorial snow can take thousands of years to disappear. When it does, the groundwater disappears as well, vaporizing directly into the atmosphere, Arvidson said.

"What you end up with is this beautiful stratigraphy, and the crust is this calcium sulfate-enriched material that [Spirit] has broken through."

The top layer of crust Spirit has been studying was likely laid down during the most recent equatorial snowfall, which happened tens of thousands of years ago.